Until you see one, it’s hard to imagine the size and power of the world’s most impressive waterfalls. Six million cubic feet of water—enough to fill more than sixty Olympic-sized swimming pools—goes over Niagara Falls, between New York and Canada, every minute. That’s enough to produce four million kilowatts of electricity, enough to power half of Las Vegas on a hot summer night. But what if the world’s biggest waterfall wasn’t big and noisy and a fancy-pants honeymoon resort? What if no one ever saw it, because it’s miles beneath the North Atlantic Ocean?

Scientists have discovered a series of enormous cataracts beneath the world’s oceans, most the result of temperature differentials. Cold water is denser than hot water (its molecules are bouncing around less, so they need less space), so it tends to sink. When cold-water basins pour into slightly warmer ones, their water plunges straight down, just like at Niagara—only slightly slower, much more quietly and almost invisibly, since it all happens underwater.

The world’s biggest underwater waterfall is located in the Denmark Strait between Greenland and Iceland. Arctic water from the Greenland Sea drops precipitously into the depths of the (slightly less chilly) Irminger Sea.

The amazing thing about the Denmark Strait cataract is that it dwarfs anything you’d see above the waves. Its water drops almost 11,500 feet, more than three times the height of Angel Falls in Venezuela, normally considered Earth’s tallest waterfall. And the amount of water it carries is estimated at 175 million cubic feet of water per second. That’s equivalent to almost two thousand Niagaras at their peak flow.

But these are tough times for undersea waterfalls. A 2008 Barcelona University study of the undersea waterfall in the Gulf of Lion—one of just three known to exist in the Mediterranean—found that global warming is slowing it down. That’s bad news for species that rely on these cataracts to circulate water of varying salinities and temperatures—and for human industries, like deep-sea shrimp fisheries, that rely in turn on those species.